The majority of modern day helicopters embody a single main rotor assembly and an exposed tail rotor assembly (noted exceptions being the Aerospatiale helicopters employing a fenestron tail structure and the McDonnell Douglas helicopters employing the NOTAR.TM. antitorque device). The exposed tail rotor assembly has proven to be a relatively efficient and reliable means for providing lateral thrust to counteract the fuselage induced torque generated by the main rotor assembly and to provide yaw directional control of the helicopter in hover, transitional, low, and high speed flight regimes.
Exposed tail rotor assemblies, however, present disadvantages from both an aerodynamic and non-aerodynamic point of view. First and foremost, exposed tail rotor assemblies present significant safety hazards during ground operations, i.e., system run-up, hovering, taxing, and/or parking operations. The exposed tail rotor assembly poses a severe danger to adjacent personnel during ground operations. Personnel have been killed or injured by inadvertent contact with the rotating tail rotor blades of an exposed tail rotor assembly. The operating exposed tail rotor assembly also poses a hazard to other equipment located in areas of helicopter ground operations. In addition, exposed tail rotor assemblies are vulnerable to damage from objects circulated by the slip stream of the main rotor assembly.
The exposed tail rotor assembly also presents problems during helicopter flight operations such as takeoffs, landings, and or maneuvering in confined areas where care must be taken to prevent inadvertent strikes of the exposed tail rotor assembly with terrain features such as power lines, buildings, fences, trees, and bushes. Numerous military applications and some civilian applications require nap of the earth (NOE) flying, sometimes at night or in reduced visibility weather. Flying in such conditions requires extra care to prevent inadvertent strikes of the exposed tail rotor assembly with such terrain features. Moreover, the exposed tail rotor assembly is a significant noise source in both civilian operations where it is an annoyance factor and in military operations where it is a factor in detection, identification, location, and/or tracking of the helicopter.
In addition, the aerodynamic efficiency of exposed tail rotor assemblies is degraded by various factors arising out of the very nature of such assemblies. An exposed tail rotor assembly is not generally utilized to provide the total yaw stability in higher speed flight regimes due to drag effects and induced stresses acting on the tail rotor blades. Instead, an aerodynamically-configured vertical stabilizer is incorporated in the configuration of the helicopter empennage to provide a portion of the required yaw stability in higher speed flight regimes. The exposed tail rotor assembly, however, still provides an observable contribution to the total aerodynamic drag in such flight regimes.
To provide the antitorque thrust (lateral lift) required for hover operations and yaw maneuvers during transitional, low, and high speed flight regimes, the typical exposed tail rotor assembly has large diameter tail rotor blades (to reduce the engine power required by the tail rotor assembly to develop such thrust). The tail rotor assembly must be mounted on the vertical stabilizer to provide the necessary ground clearance for the tail rotor blades. Such an arrangement, however, results in aerodynamic interference between the vertical stabilizer and the exposed tail rotor assembly (stabilizer blockage) that reduces the aerodynamic efficiency of the exposed tail rotor assembly. This arrangement may also interfere with the aerodynamic functioning of the vertical tail structure in higher speed flight regimes. In addition, such an arrangement creates an induced roll moment about the longitudinal axis of the helicopter.
Furthermore, an exposed tail rotor assembly is generally a mechanically complex and fragile apparatus that is subjected to severe operating stresses and dynamic phenomena such as relative wind, main rotor assembly and fuselage slip streams and vortices that reduce the operating efficiency thereof. Exposure to such operating conditions tends to limit the overall useful lifetime of an exposed tail rotor assembly such that the costs associated with more frequent maintenance/overhaul are increased. In addition, exposed tail rotor assemblies are subjected to increased blade loading effects during flights at increased sideslip angles, which tends to restrict the effective operating envelope with respect to sideslips for helicopters having exposed tail rotor assemblies.
A helicopter embodying a fenestron or ducted fan antitorque device in the empennage structure provides several aerodynamic and non-aerodynamic advantages over the conventional helicopter configuration. An operating ducted fan antitorque device does not present a significant hazard to adjacent personnel or equipment. Furthermore, the empennage structure effectively shields the ducted fan from damage induced by external objects.
Acoustically, a ducted fan antitorque device provides several advantages over an exposed tail rotor assembly. The duct shields the fan noise from the far field observer, especially in the direction of flight. The thrust augmentation provided by the duct substantially reduces the loading noise component from the fan blades--a primary noise source at the lateral aspects of the helicopter. Further, the acceleration induced by the duct in the mass airflow influx into the fan assembly substantially reduces the turbulence of the mass airflow influx which may be a significant noise source from the fan blades. In addition, a relatively large number of blades increases the harmonic frequencies generated by the fan assembly to such an extent that atmospheric absorption and ground attenuation may provide significant attenuation of the fundamental and higher harmonics, noise yet not so high as to be in the range of highest hearing sensitivity. Typically, the first few harmonics of an exposed tail rotor assembly are so low as to allow for long range propagation while the relatively large number of blades of the Aerospatiale fenestron (generally 11 or 13) places the second and first harmonics, as well as higher harmonics, in the frequency range of the highest hearing sensitivity.
Aerodynamically, a ducted fan antitorque device may be effectively off-loaded in higher speed flight regimes, thereby providing a reduction in total aerodynamic drag in these flight regimes. The vertical stabilizer does not aerodynamically interfere with the operation of a ducted fan antitorque device. The tail rotor assembly of the ducted fan antitorque device is not exposed to external dynamic phenomena such that the overall lifetime of the ducted fan tail rotor assembly is improved, with the concomitant decrease in maintenance requirements. A ducted fan antitorque device effectually reduces stresses experienced by tail rotor blades during sideslip flight regimes, thereby expanding the operating envelope of helicopters embodying ducted fan antitorque devices. For equivalent rotor defined apertures, the aerodynamic efficiency of the ducted fan antitorque device is greater than that of an exposed tail rotor assembly such that the ducted fan antitorque device may be downsized for incorporation in the empennage structure while still providing substantially equivalent aerodynamic performance. This reduced diameter allows the device to be mounted at a lower waterline, eliminating the induced roll moment about the longitudinal axis of the helicopter as experienced by helicopters utilizing exposed tail rotor assemblies.
Aerospatiale has produced several lines of helicopters such as the Dauphin and Gazelle that include an empennage structure embodying a ducted fan antitorque device and a vertical stabilizer in combination to provide antitorque thrust and yaw directional control for a helicopter. The ducted fan antitorque devices of these helicopters have an duct axis that is substantially perpendicular to the vertical plane of symmetry of the helicopter, i.e., the tail rotor blade plane is parallel to the vertical plane. The transverse thrust developed by these antitorque devices is sufficient to provide the necessary antitorque force and yaw directional control in the hover, translational, low, and high forward speed flight regimes. The empennage structure of these helicopters includes a vertical stabilizer that is aerodynamically configured to provide lateral thrust for antitorque and yaw stability at higher forward speeds.
U.S. Pat. No. 4,809,931, issued to Aerospatiale, discloses that such prior art empennage structures do not provide any pitch stability, particularly at higher forward speeds. The '931 patent teaches that a horizontal stablizing surface is required to provide an empennage structure that provides both static and dynamic yaw and pitch stability as well as the counterbalancing antitorque thrust. The '931 patent further teaches that this type of empennage structure is disadvantageous in that it results in a substantial increase in overall structural weight of the helicopter.
Another prior art empennage structure embodying a ducted fan antitorque device is described in the '931 patent, this prior art empennage structure replacing the vertical and horizontal stabilizers with two aerodynamic surfaces. The '931 patent teaches that the two aerodynamic surfaces extend above a horizontal plane passing through the top of the housing of the ducted fan antitorque device, and that the mean planes of the aerodynamic surfaces are disposed symmetrically with respect to each other about the vertical plane passing through the housing to define a "V" empennage. These aerodynamic surfaces are described as being configured to provide antisymmetrical aerodynamic lift profiles. The '931 patent teaches that such an empennage configuration has not achieved the advantageous results expected.
The subject matter described and claimed in the '931 patent is a helicopter empennaqe structure embodying a ducted fan antitorque device that simultaneously provides the antitorque force and static and dynamic stability about the yaw and pitch axes. The '931 patent teaches that the mean plane of the ducted fan antitorque housing is slanted with respect to the vertical plane of symmetry of the helicopter in an angular range of 0.degree. to 45.degree.. Two aerodynamic surfaces are joined in combination at the top of the housing to form a "V" empennage extending above the horizontal plane passing through the top of the housing. The '931 patent teaches that the two aerodynamic surfaces are disposed in several different embodiments wherein the respective mean planes of the aerodynamic surfaces extend disymmetrically with respect to the vertical plane of symmetry of the helicopter. The mean plane of the two aerodynamic surfaces are described as forming predetermined angles, selected from a defined range of angles, with respect to the horizontal and vertical, respectively.